Parker O-Ring Handbook.pdf

misapplication might be greatly reduced. ASTM Designation
D471 (Standard Method of Test for Change in Properties
of Elastomeric Vulcanizates Resulting from Immersion in
Liquids) states: “In view of the wide variations often present
in service conditions, this accelerated test may not give
any direct correlation with service performance. However,
the method yields comparative data on which to base judgment
as to expected service quality and is especially useful
in research and development work.”
2.17 Specifi cations
Specifi cations are important, but so is progress. Therefore,
even though it may be more diffi cult to prepare, a performance
specifi cation is recommended. This allows new developments
and improvements to be adopted without any appreciable
effect on the specifi cation.
Avoid specifying how to compound materials or process
compounds. Let the seal manufacturer examine the performance
desired. A vendor should be allowed to supply his
best solution to a problem. It is not only possible, but also
probable that a well-qualifi ed supplier knows of materials
and/or processes that will solve the problem and one should
be permitted to use them.
It must be recognized that physical properties provide a means
of screening new materials for an application by setting realistic
minimums. These can be established when experience with
certain properties gives a good indication of the suitability
of a new material for the application. These properties also
permit control of a material after it has proven satisfactory for
an application. Therefore, a brief discussion of the main points
Volume Change — Percent
6
4
2
0
-2
0
Parker O-Ring Handbook
Variance in O-Ring Volume Change with Cross-Section W
MIL P-5516 Nitrile
Immersed in
MIL-H-5606 (J-43)
W
0.210
0.139*
0.103*
Figure 2-30: Variance in O-ring Volume Change With Cross-Section W
*Averages of many samples
Volume Change — Percent
10
8
6
4
2
that should be considered when preparing the physical and
chemical test portions of a specifi cation follows. The discussion
is in the order that specifi cations are usually written and tests
carried out. There are three major points that must always be
considered when preparing any specifi cation. These are:
1. Different size parts give different results (see Figure 2-
30). All parts with varying cross section or shape will not
meet specifi c properties set up on another particular part
or on test specimens cut from a standard 6" x 6" x 0.075"
test sheet. Therefore, always designate the actual parts on
which the tests are to be conducted for both qualifi cation
and control. For example, call for a particular size O-ring
if the standard ASTM 6" x 6" x 0.075" test platens are not
to be used.
2. Always use standard hardness discs (1.28" dia. = 1 in² by
1/4" thick) or 6" x 6" x 0.075" sheets plied up to a minimum
thickness of 1/4" to determine durometer hardness. It has
been almost impossible to obtain reliable and reproducible
hardness readings on seals with curved surfaces and
variable cross sections (such as O-rings). This problem
has plagued the industry for years and is acknowledged
in both specifi cation and test standards. For example:
ASTM Method D2240, paragraph 6-1 states: “A suitable
hardness determination cannot be made on a rounded,
uneven, or rough surface.”
3. It is recommended that standard test methods be used
whenever possible. Consider the case of the deviation
from the standard methods of taking instantaneous
durometer readings. Occasionally, fi fteen or thirty second
delayed durometer readings are specifi ed. A delayed
W
0.139
0.103
0.210
0.070
0.060
0.070*
0
1 2 3
1 2 3 4 5 6
Immersion Time at 70°C (158°F) — Weeks
0.275
Butyl Rubber
Immersed in
Skydrol 500A
Parker Hannifi n Corporation • O-Ring Division
2360 Palumbo Drive, Lexington, KY 40509
Phone: (859) 269-2351 Fax: (859) 335-5128
www.parkerorings.com
Basic O-Ring Elastomers
2-29